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9/11/2016 Cisco E­Learning for ICND1 v2.0 https://ondemandelearning.cisco.com/cisco­sc/icnd1#/sections/8/pages/6 1/4 Reserved IPv4 Addresses Section 8: Understanding the TCP/IP Internet Layer Certain IP addresses are reserved and cannot be assigned to individual devices on a network. Reserved IP addresses include a network address, which is used to identify the network itself, and a broadcast address, which is used for broadcasting packets to all of the devices on a network. Network Address The network address is a standard way to refer to a network. An IP address that has binary 0s in all of the host bit positions is reserved for the network address. For example, in a Class A network, 10.0.0.0 is the IP address of the network containing the host 10.1.2.3. All hosts in 10.0.0.0 will have the same network bits. The IP address 172.16.0.0 is a Class B network address, and 192.16.1.0 is a Class C network address. A router uses the network IP address when it searches its IP routing table for the destination network location. In a Class B network address, the first two octets are the network portion. The last two octets contain 0s because those 16 bits are for host numbers and are used for devices that are attached to the network. In the IP address 172.16.0.0, the first two octets are reserved for the network address and are never used as an address for any device that is attached to it. An example of an IP address for a device on the network is 172.16.16.1. In this example, 172.16 is the network address portion and 16.1 is the host address portion. Local Broadcast Address If an IP device wants to communicate with all of the devices on the local network, it sets the destination address to all 1s (255.255.255.255) and transmits the packet. For example, hosts that do not know their network number and are asking a server for it may use this address. The local broadcast is never routed beyond the local network (subnet). 9/11/2016 Cisco E­Learning for ICND1 v2.0 https://ondemandelearning.cisco.com/cisco­sc/icnd1#/sections/8/pages/6 2/4 Directed Broadcast Address The broadcast IP address of a network is a special address for each network that allows communication to all of the hosts in that network. To send data to all of the hosts in a network, a host can send a single packet that is addressed to the broadcast address of the network. The broadcast address uses the highest address in the network range. This is the address in which the bits in the host portion are all 1s. For network 10.0.0.0, with 8 network bits, the broadcast address would be 10.255.255.255. This address is also referred to as the directed broadcast. Assuming a hypothetical network where every Class A IP host address was in use, a ping to 10.255.255.255 would receive a response from all 16,777,214 hosts. For the network address 172.16.0.0, the last 16 bits make up the host field (or host part of the address). The broadcast that would be sent out to all of the devices on that network would include a destination address of 172.16.255.255. 9/11/2016 Cisco E­Learning for ICND1 v2.0 https://ondemandelearning.cisco.com/cisco­sc/icnd1#/sections/8/pages/6 3/4 The directed broadcast address can be routed over your company's intranet and over the Internet. In the 1990s, a popular DoS attack referred to as a Smurf used directed broadcasts to send so much traffic to an intended victim that they could not send or receive any legitimate traffic. For this reason, IOS defaults to disallowing directed broadcasts. This capability can be restored with the ip directed-broadcast command in global configuration mode. It is a best practice to leave directed broadcasts disabled unless you have a specific use case. Routers began using the no ip directed-broadcast command as a platform default starting with Cisco IOS version 12.0. Local Loopback Address A local loopback address is used to let the system send a message to itself for testing. The loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate with one another. A typical local loopback IP address is 127.0.0.1. On a Microsoft Windows 7 host, you can ping any IP address in the 127.0.0.0/8 range. Network ID The network portion of an IP address is also referred to as the network ID. This is important because most hosts on a network can directly communicate only with devices in the same network. If the hosts need to communicate with devices that have interfaces that are assigned to another network ID, they must go through a network device that can route data between the networks. This is true even when the devices share the same 9/11/2016 Cisco E­Learning for ICND1 v2.0 https://ondemandelearning.cisco.com/cisco­sc/icnd1#/sections/8/pages/6 4/4 physical media segment. The network ID cannot be assigned to a host. For example, 10.0.0.0 cannot be assigned because it is the network ID for that Class A network. A network ID enables a router to transmit an IP packet onto the appropriate network segment. Up Next: Private Versus Public IPv4 Addresses table for the destination network location. In a Class B network address, the first two octets are the network portion. The last two octets contain 0s because those 16 bits are for host numbers and are used for devices that are attached to the network. In the IP address 172.16.0.0, the first two octets are reserved for the network address and are never used as an address for any device that is attached to it. An example of an IP address for a device on the network is 172.16.16.1. In this example, 172.16 is the network address portion and 16.1 is the host address portion. Local Broadcast Address If an IP device wants to communicate with all of the devices on the local network, it sets the destination address to all 1s (255.255.255.255) and transmits the packet. For example, hosts that do not know their network number and are asking a server for it may use this address. The local broadcast is never routed beyond the local network (subnet). 9/11/2016 Cisco E­Learning for ICND1 v2.0 https://ondemandelearning.cisco.com/cisco­sc/icnd1#/sections/8/pages/6 2/4 Directed Broadcast Address The broadcast IP address of a network is a special address for each network that allows communication to all of the hosts in that network. To send data to all of the hosts in a network, a host can send a single packet that is addressed to the broadcast address of the network. The broadcast address uses the highest address in the network range. This is the address in which the bits in the host portion are all 1s. For network 10.0.0.0, with 8 network bits, the broadcast address would be 10.255.255.255. This address is also referred to as the directed broadcast. Assuming a hypothetical network where every Class A IP host address was in use, a ping to 10.255.255.255 would receive a response from all 16,777,214 hosts. For the network address 172.16.0.0, the last 16 bits make up the host field (or host part of the address). The broadcast that would be sent out to all of the devices on that network would include a destination address of 172.16.255.255. 9/11/2016 Cisco E­Learning for ICND1 v2.0 https://ondemandelearning.cisco.com/cisco­sc/icnd1#/sections/8/pages/6 3/4 The directed broadcast address can be routed over your company's intranet and over the Internet. In the 1990s, a popular DoS attack referred to as a Smurf used directed broadcasts to send so much traffic to an intended victim that they could not send or receive any legitimate traffic. For this reason, IOS defaults to disallowing directed broadcasts. This capability can be restored with the ip directed-broadcast command in global configuration mode. It is a best practice to leave directed broadcasts disabled unless you have a specific use case. Routers began using the no ip directed-broadcast command as a platform default starting with Cisco IOS version 12.0. Local Loopback Address A local loopback address is used to let the system send a message to itself for testing. The loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate with one another. A ty